1. Field of the Invention
This invention relates to a method and apparatus for evaluating the sagging properties of a sealant with thixotropy included.
2. Prior Art
A sealant used in the car manufacturing industry is imparted with thixotropic properties so that the sealant can maintain its shape upon application until being cured. The sealant is used in the process of fitting a glass sheet, such as a windshield, to an automotive body in which process the sealant is applied in the form of a triangle-shaped bead to the window frames and then the windshield is fitted thereon. The reason for applying the sealant in a form of the triangle shaped bead rather than a round shape in section is that the sealant can secure fixedly to the windshield on the window frame in a smaller amount. More specifically, the portion of the window frames where the windshield is to be fixed is a groove that is concave in section. If a sealant rounded in section is applied onto the groove, it makes contact with the upper walls of the groove and thus fails to be applied into the groove completely, leading to the necessity of pressing the sealant into the groove and application of an extra amount of the sealant so as to maintain the windshield in position sufficiently. On the contrary, a sealant in the shape of a triangle can be put into the groove by inserting it from the tip or corner until it reaches the bottom of the groove and thus can be applied in larger amounts than the rounded one, thereby obtaining a sufficient sealing effect. Therefore, a sealant is required to be imparted with thixotropy to maintain this shape when being applied.
The criterion of thixotropy to be imparted differs depending upon the usage of a sealant. There is one conventional method known as "Car Manufacturing Specification Evaluation Method" stipulated as JASO M 338-89 by the Japanese Automobile Standards Organization which can evaluate the sagging conditions of a sealant.
In this method, a sample sealant (32) is coated in the shape of a triangulated bead onto a glass sheet (31) and held vertically in a standing position for 30 minutes, followed by observing the level of sagging of the sealant (32) as shown in FIGS. 2a and 2b.
However, this method fails to provide precise evaluation because of not conforming to a real automotive production line where a sealant is automatically applied to automotive bodies by means of a plurality of nozzles, but is kneaded before arriving at the site where the nozzles are located. The sealant is supplied from a vessel, such as a drum, by means of a pump through a plurality of pipes and, thus, is subjected to shear stress until being discharged from the nozzles, effecting adversely the sagging conditions. The sealant once affected by shear stress is seemingly in the softened state and prone to sag upon application to an automotive body. However, the evaluation of the above-mentioned method results from a sample sealant which is not subjected to kneading and thus, does not encounter sagging as experienced in a real automotive production line. Therefore, this method can not evaluate the sagging conditions of a sealant in conformity with a real production line.
There is another conventional method for evaluating the sagging conditions of a sealant known as "Developing Department Shear Method". In this method, a sealant (35), in a predetermined amount, is coated onto a side wall of a polyethylene cup (33) as shown in FIG. 3a. The sealant is then applied with shear stress using a spatula (34) a predetermined number of times for a predetermined period of time and thereafter, followed by examining the shape of the sealant after pulling out the spatula therefrom as shown in FIG. 3b thereby evaluating the sagging conditions of the sealant (35). However, this method also is accompanied with a problem because when an operator applies shear stress on the sealant (35) manually using the spatula (34), it is extremely difficult to apply It repeatedly with the same degree of shear stress even though a single operator conducts the method. Furthermore, in the case where shear stress is applied by more than two operators, the resulting data are scattered due to the different criteria depending upon each operator.